Two-stroke gasoline engines have been used as a power source of portable power working machines such as brush cutters, chain saws or the like. In a two-stroke internal combustion engine of this type, a combustion chamber is scavenged by a flow of air-fuel mixture pre-compressed in a crank chamber. More specifically, as the piston ascends, the air-fuel mixture is introduced into the crank chamber; and pre-compressed by the descending piston. Then, during the scavenging stroke, the pre-compressed air-fuel mixture is introduced into the combustion chamber to force the waste combustion gas (exhaust gas) out of the combustion chamber and replace it.
As such, the two-cycle engines are configured to scavenge the combustion chamber by using flows of air-fuel mixture, and therefore involve the problem of “blow-by”. That is, a part of the air-fuel mixture, introduced into the combustion chamber and having not burnt, is undesirably discharged away from the combustion chamber together with a combustion gas. This “blow-by” phenomenon makes it difficult to take effective measures for emissions cut of two-stroke engines.
To control the “air-fuel mixture blow-by” phenomenon, a stratified scavenging technique has been proposed in Document 1 (Japanese Laid-open Publication No. 2001-55958) and Document 2 (Japanese Laid-open Publication No. 2000-170611). This stratified scavenging technique is called “initial scavenging by air” as well. Documents 1 and 2 disclose a carburetor including an air-fuel mixture passage for generating air-fuel mixture and an air passage through which fuel-free air (air not containing fuel) passes. In the carburetor disclosed in Documents 1 and 2, air cleaned by an air cleaner common to both passages is supplied to the air-fuel mixture passage and the air passage.
A typical air cleaner for the engine of this kind, as discussed as a conventional example in Document 2 (see FIG. 9 of this document), has a single tubular air intake in communication with both the air-fuel mixture passage and the air passage of the carburetor to supply air to both the air-fuel mixture passage and the air passage through the common air intake. However, as pointed out in Document 2, air-fuel mixture partly pushed back into the air cleaner due to “blow-back” from the air-fuel mixture passage of the carburetor flows into the air passage, thereby causing the fuel to mix with fresh air for use in initial scavenging. This will invite the problem that an unburnt component is discharged from the engine together with combustion gas. Needless to say, this is one of technical problems to be improved in the recent society under an increased request for cleaning or purifying exhaust gas.
To cope with this problem, Document 2 proposes to provide a baffle plate on the common air intake of the air cleaner. More specifically, for the purpose of preventing the air-fuel mixture from flowing into the air passage even though it is pushed back into the air cleaner due to “blow-back” from the air-fuel mixture passage of the carburetor, Document 2 proposes to partition the air intake of the air cleaner communicating with the carburetor into a first air intake communicating with the air-fuel mixture passage of the carburetor and a second air intake communicating with the air passage, using a baffle wall between the first and second air intakes.